Specific, metabolism-driven transporters in excretory epithelia and barrier tissues play a important role in determining xenobiotic uptake, distribution and excretion. Along with xenobiotic metabolizing enzymes, these transporters are our first line of defense against chemical toxins, but they do not distinguish between toxic chemicals and therapeutic drugs. Recent focus has been on xenobiotic transport at the blood-brain and blood-cerebrospinal fluid (CSF) barriers, where we are identifying and characterizing the transporters present and beginning to explore mechanisms that regulate expression and function. The primary structure responsible for the blood-brain barrier is the non-fenestrated brain capillary endothelium. Although originally thought to present a passive, anatomical barrier to xenobiotics, it is now clear that multispecific xenobiotic transporters are a critical feature of the barrier. We are using isolated brain capillaries from rat, pig and mouse along with confocal microscopy to both functionally map transporters involved in the selective barrier and examine their regulation by hormones, xenobiotics and disease. Using this experimental system, we have defined a role for the multidrug resistance associated protein isoform2 (Mrp2) in barrier function and demonstrated the involvement of both Mrp2 and p-glycoprotein in the low permeability of HIV-protease inhibitors into the CNS. Moreover, we have demonstrated modulation of drug efflux transporter activity in brain capillaries by 1) ligands that activate several nuclear receptors expressed in the tissue, and by 2) the polypeptide hormone, endothelin-1 (ET-1). Both signaling pathways may be useful in selectively altering barrier function to facilitate pharmacotherapy. The choroid plexus is responsible for removal of potentially toxic xenobiotics and metabolites from the cerebrospinal fluid (CSF). In this tissue our work has focused primarily on functionally mapping transporters that mediate concentrative transport of anionic xenobiotics and metabolic wastes from CSF to blood. Using selective inhibitors and tissue from rats and an Oat3-null mouse model, we have identified Oat3 as one major contributor to organic anion uptake. We have also functionally defined several routes of uptake that at present lack a olecular correlate. Thus, concentrative organic anion uptake cannot be accounted for by the transporters known to be expressed in the tissue. Finally, we are also using confocal imaging to explore for the first time efflux mechanisms at the blood side of the tissue, where transport appears to be driven by membrane potential and by ATP splitting.